An engine valve used for an internal combustion engine such as an automotive engine, particularly an exhaust valve, is exposed to high temperature. Accordingly, the engine valve is configured such that the stem thereof is hollow, and metallic sodium is enclosed in the hollow area of the stem. Metallic sodium to be enclosed is solid at room temperature. However, the melting point of metallic sodium is about 98° C. Thus, the metallic sodium is liquefied at relatively low temperature. Accordingly, when the valve is warmed by activating engine, the metallic sodium become liquid and is shaken in the valve stem owing to a vertical movement of the valve. Thereby, heat transferred from a combustion chamber to the valve head is transferred through the valve stem so as to dissipate to a water jacket of a cylinder head through a valve guide contacting with the valve stem. This prevents overheat of the combustion chamber. Moreover, since the specific gravity of metallic sodium is 0.97, i.e. less than that of water, the metallic sodium filled in the valve can contribute to light-weighting of the entire valve.
Metallic sodium has a strong deoxidizing action so that it deoxidizes water to generate hydrogen while the metallic sodium per se changes to sodium hydroxide. Therefore, in order to prevent the oxidization of metallic sodium and in order to be stably preserved for a long period of time, the metallic sodium is stored under the condition where it is immersed in an organic solvent, such as kerosene, liquid paraffin (a mixture of relative long-chain saturated hydrocarbons, having a boiling point of several hundred degrees) or the like, with blocking water and air. Further, each of kerosene and liquid paraffin has a less specific gravity than metallic sodium, so that metallic sodium is securely blocked from water and air without floating on the surface of such solvents.
Filling the valve stem of the engine valve with such metallic sodium stored in the organic solvent is performed by taking out a bulk body of metallic sodium immersed in the organic solvent from the solvent, melting the metallic sodium, pouring the metallic sodium in a melting state into the stem of the engine valve, and then cooling the valve (Patent Document 1). Or, it is performed by ejecting melted metallic sodium linearly into a hydrocarbon liquid to solidify in a rod shape, and thereafter inserting the metallic sodium rod into the hollow area of the engine valve to be sealed (see FIGS. 2 and 4 of Patent Document 2).